Exhaust emissions regulations for internal combustion engines have become more stringent over recent years. Therefore, the use of exhaust after-treatment systems on engines to reduce harmful exhaust emissions is increasing. Typical exhaust after-treatment systems include any of various components configured to reduce the level of harmful exhaust emissions present in the exhaust gas. Emission requirements vary according to engine type. For example, emissions tests for compression-ignited engines (e.g., diesel-powered engines) typically monitor the concentration of carbon monoxide, nitrogen oxides (NOx), and unburned hydrocarbons (UHC) that are released from the tail-pipe to make sure that the concentrations of such compounds leaving the tail-pipe are within certain emissions standards. With regard to reducing NOx emissions, NOx reduction catalysts, including selective catalytic reduction (SCR) components, have been used to convert NOx (NO and NO2, in some fraction) to N2 and other compounds.
Conventional SCR components utilize a diesel exhaust fluid (DEF) (e.g., aqueous urea, ammonia, etc.) as a reagent to reduce the NOx. When the proper amount of ammonia is present in the exhaust gas stream at the SCR catalyst, the ammonia is consumed (oxidized) in the reaction and the NOx is reduced. However, accurately controlling the amount and dispersion of ammonia in the exhaust gas stream can be difficult. While anhydrous ammonia can be used, it is toxic and difficult to safely store. Aqueous ammonia or urea is often used as the reductant because such compounds are safer to store than anhydrous ammonia. Urea, which consists of two primary amine groups joined by a carbonyl, is the safest to store; however, the urea must be thermally decomposed into ammonia and vaporized before being oxidized as a reagent in the SCR catalyst.
Also, since the selectivity and overall reactivity of catalytic reactions (e.g., SCR) is largely dependent on the extent of dispersion of the reactants and reagents across the surface of the catalyst (e.g. stoichiometric ratio), sufficiently mixing and diffusing the DEF into the exhaust gas stream is critical to the success of SCR. Thus, aftertreatment systems that use ammonia-facilitated SCR components often include a DEF pumping system that injects the DEF into the exhaust gas stream. Such pumping systems, however, often suffer from poor start-up performance. For example, if the engine (and therefore the aftertreatment system) has not been operating for a period of time, segmented columns of air form throughout the DEF lines of the pumping system, potentially causing the pump to dead-head (i.e. produce no net flow) upon actuation (e.g., upon engine ignition and pump initialization). Various conventional systems have been implemented that attempt to counteract this problem by priming the pumping system. However, these priming systems often include multiple lines extending to and from the DEF source, which adds cost and complexity to the aftertreatment system.